Method for evaluating a measuring signal and corresponding circuit arrangement

ABSTRACT

In order to measure the current in a shunt, the voltage drop in the resistor of said shunt is evaluated as a measurement of the current. Normally, galvanic separation of the measuring device is required for high-potential shunts. According to the invention, a digital measuring signal provides the clock pulse for modulated supply current for the measuring device after an A/D conversion. The modulated supply current is also used as a carrier for the measuring information. In the associated evaluation device, an A/D converter ( 3 ) and a modulation unit ( 4 ) are arranged downstream from an amplifier ( 2 ) for the voltage signal which is picked off from the shunt ( 1 ). The modulated measuring signal is transmitted to an evaluation device ( 5 )

[0001] The invention relates to a method for evaluation of a measurement value as claimed in the precharacterizing clause of patent claim 1. The invention also relates to the associated circuit arrangement for carrying out the method, for use for current measurement on a shunt, in which the voltage drop is evaluated as a measure of the current.

[0002] Especially for current measurement, it is normal to use a shunt, which is essentially in the form of a defined resistance, and to indicate the voltage drop across this resistance as the current value. A shunt such as this is frequently at a potential higher than ground potential.

[0003] Current measurement on a shunt which is at a potential is known. The LM 3822 module from the National Semiconductor Company produces a PWM signal after analog/digital conversion, for current measurement (see Internet www.Power.National.com).

[0004] When a shunt such as this is at high potential, DC isolation is generally specified for further processing of the measurement signals. Optocouplers, for example, are used for this purpose. Since the voltage drop is generally amplified by operational amplifiers or the like, an auxiliary power supply is also necessary, at least for the amplifier. Thus, in practice, it is necessary to accept significant complexity for the auxiliary power supply on the one hand and for the voltage decoupling on the other hand, for this purpose.

[0005] A linear current measurement circuit is known from DE 199 10 755 A1, especially for a motor circuit in which an input circuit which is at high voltage and has an amplifier and an A/D converter supplies a digital signal with a sequence of current pulses, which are converted back to an analog signal at ground potential.

[0006] Furthermore, FR 2 541 777 A discloses an apparatus for signal transmission by means of transformers between a primary section in the low-voltage range and a secondary section in the high-voltage range, in which a power supply is provided in the primary section, which transmits the energy which is required for operation of the secondary section by means of a transformer, in the form of power pulses from the primary section to the secondary section. In this case, the information is transmitted by the duty ratio of the periodic power pulse pauses.

[0007] Against the background of the prior art, the object of the invention is to minimize the technical complexity for current measurement at a potential.

[0008] For a method of the type mentioned initially, the object is achieved by the features of patent claim 1. An associated circuit arrangement, especially for current measurement on a shunt, is specified in patent claim 7. Advantageous developments of the method and of the associated arrangement are the subject matter of the respective dependent claims.

[0009] In the invention, the measurement value, which is at high potential, is transferred to ground potential by modulation of its own power supply, which is at a high-voltage potential. The power supply for detection of the measurement signal and for the transmission of the measurement value are thus combined. The DC isolation which was still necessary until now can thus be avoided. In particular, this measure makes the solution according to the invention cheaper than the procedure according to the prior art.

[0010] The invention results in simultaneous use of the power supply for the unit for data transmission by modulation of the supply current. It is thus possible to carry out the measurement at high potential, and to transmit the measurement value as a modulated supply current.

[0011] The invention is particularly suitable for current measurement by means of a shunt which is at a potential. However, the invention can also be used for any other measurement tasks in which any desired measurement values are at a relatively high potential.

[0012] Further details and advantages of the invention can be found in the following description of the figures with reference to the drawing and in conjunction with the patent claims. In the figures:

[0013]FIG. 1 shows the principle of a current measurement device, in which the measurement values are transmitted by modulation of its own power supply,

[0014]FIG. 2 shows a modification of FIG. 1, with differential modulation.

[0015] Identical units have the same reference symbols in the figures. Some of the figures are described jointly.

[0016] In the figures, L1 denotes a phase of a line which is at a potential. N denotes the neutral conductor at ground potential. A load 10 is connected to L1 and N. There may be two further phases L2 and L3 in an equivalent manner.

[0017] A shunt 1 with a resistance value R_(S) is connected in the line L1 which is at a potential. The voltage U1 across the shunt 1 is tapped off and is supplied to an amplifier 2. The amplifier 2 is followed by an analog/digital (A/D) converter 3, which emits a digital signal.

[0018] The digital signal from the A/D converter 3 is used as a modulation clock for a modulation unit 4, which is also used for supplying power to the units 2 and 3. In order to produce a supply current, the modulation unit 4 is connected to the live lines, that is to say to the phase conductor L1 and to the neutral conductor N.

[0019] The supply current is now modulated by the modulation unit 4 and is passed via resistors 6 and 7 with resistance values R_(v) and R_(m) to the neutral conductor N, which is at ground potential. The resistors 6 and 7 are used as a voltage divider, with one branch being connected via the resistor 7 to an evaluation unit 5, which is at ground potential. The evaluation unit 5 is, for example, a microcontroller, via which the measurement value is emitted.

[0020] The supply current can be modulated for example by on/off keying in the modulation unit 4. A first switch 41 and a second switch 42, for example, are provided for this purpose and are expediently in the form of semiconductor switches. The two switches S1 and S2 are opened and closed in antiphase, with the operating frequency depending on the value of the measurement signal to be transmitted. For this purpose, the modulation unit 4 has a switch controller 40, which is driven by the A/D converter 3 and emits the antiphase switch signals to the switches 41 and 42.

[0021] The evaluation of the measurement value comprises a frequency measurement of the modulation clock. The clock signal is recovered via the voltage drop of the supply current, which is modulated with the measurement information, across the resistor 7, which has a defined resistance value R_(M). A signal with the original measurement value is thus produced.

[0022] The modulation unit 4 also contains a power supply unit 44 with a rectifier circuit. The rectifier circuit, which is indicated symbolically in FIG. 1, produces an auxiliary current which supplies current to those components which are at a potential, in particular such as the amplifier 2 and the A/D converter 3.

[0023] In FIG. 2, the arrangement shown in FIG. 1 has been modified for differential modulation of the measurement signals. To this end, there are two voltage dividers comprising the resistors 6, 7 and 6′, 7′ with respective resistance values R_(v1), R_(m1) and R_(v2), R_(m2), from which a difference value is passed via a differential amplifier 8 to the evaluation unit 5. [lacuna] are driven. However, apart from this, the modulation function remains the same.

[0024] The circuits which have been described with reference to the two exemplary embodiments make simultaneous use of the power supply as a unit for data transmission by the modulation of the supply current. It is thus possible in a simple manner to carry out the measurement at high potential and to transmit the measurement value in order to emit measurement values.

[0025] The described method is suitable for both DC and AC networks, in particular three-phase networks as well. In addition to use for current measurement, other applications are also possible, such as temperature measurements for devices which are at a potential. Devices such as these occur, for example, in motor vehicles, in which one pole of a load is connected by means of intelligent switches to the positive supply voltage, while the other pole is connected to ground. Information about the temperature is also transmitted in this case, as well as the current. 

1. A method for evaluation of a measurement signal, which is in the form of an analog value at a potential higher than zero potential in a measurement device which requires a supply current, and whose measurement information is transmitted after A/D conversion as a digital signal to an evaluation unit which is at ground potential, characterized in that, after the A/D conversion of the measurement signal, the digital signal which is produced provides the clock for modulation of the supply current, which thus at the same time carries out the function of the carrier for the information content of the measurement signal, with the supply current being drawn directly from the high-voltage network.
 2. The method as claimed in claim 1, characterized in that two current path elements of the supply current are modulated in antiphase.
 3. The method as claimed in claim 1, characterized in that the modulation frequency depends on the value of the measurement signal to be transmitted.
 4. The method as claimed in claim 3, characterized in that the measurement signal is transmitted in binary form, as a numerical value.
 5. The method as claimed in one of the preceding claims, characterized in that a frequency and/or period duration measurement of the supply current which is modulated with the measurement information is carried out for evaluation of the measurement signal.
 6. The method as claimed in claim 5, characterized in that the clock of the digital signal which is produced in the A/D converter is recovered for evaluation of the modulated supply current.
 7. A circuit arrangement for carrying out the method as claimed in claim 1 or one of claims 2 to 6, for use with a current measurement on a shunt, in which the voltage drop is evaluated as a measure of the current after amplification, having a shunt (1), having an amplifier (2) for the voltage signal which is tapped off across the shunt (1), having an analog/digital converter (3), and having a modulation unit (4) for supplying current to the measuring components (2, 3), with the modulation unit (4) having a power supply unit (44) for the measurement components (2, 3) and being at a high-voltage potential.
 8. The circuit arrangement as claimed in claim 7, characterized in that in that at least one resistor (7, 7′) together with an evaluation unit (5) which is associated with it and is at ground potential are provided for the modulated supply current.
 9. The circuit arrangement as claimed in claim 8, characterized in that the at least one resistor (7) has a further associated resistor (6), with the defined resistance values (R_(v), R_(m)) of the two resistors (6, 7) acting as a voltage divider.
 10. The circuit arrangement as claimed in claim 9, characterized in that two resistors (7, 7′) with defined resistance values (R_(m1), R_(m2)) and two further resistors (6, 6′) with defined resistance values (R_(v1), R_(v2)) are provided as two voltage dividers for differential modulation.
 11. The circuit arrangement as claimed in claim 9 or claim 10, characterized in that the voltages which are tapped off on the voltage dividers (6, 7; 6′, 7′) are passed via a differential amplifier (8) to the evaluation unit (5).
 12. The circuit arrangement as claimed in claim 7, characterized in that the modulation unit (4) has two switches (41, 42), which operate in antiphase and have an associated switch controller (40).
 13. The circuit arrangement as claimed in claim 7, characterized in that the modulation unit (4) has resistors (46, 47) which are driven by a modulation amplifier (45). 